See <http://www.haskell.org/pipermail/haskell/2006-September/018497.html>
On Fri, 12 Nov 2010, Petr Pudlak wrote:
On Fri, Nov 12, 2010 at 07:52:53PM +0100, Petr Pudlak wrote:
Hi, I was playing with the following example I found in D.A.Turner's paper
Total Functional Programming:
data Bad a = C (Bad a -> a)
bad1 :: Bad a -> a
bad1 b@(C f) = f b
bad2 :: a
bad2 = bad1 (C bad1)
To my surprise, instead of creating a bottom valued function (an infinite
loop), I managed to send the GHC compiler (ver. 6.12.1) to an infinite
loop. Could anybody suggest an explanation? Is this a GHC bug? Or is this
"Bad" data type so evil that type checking fails?
Thanks,
Petr
PS: The following code compiles, the difference is just in modifying "bad2"
to include an argument:
data Bad a = C (Bad a -> a)
bad1 :: Bad a -> a
bad1 b@(C f) = f b
bad2 :: (a -> a) -> a
bad2 f = bad1 (C $ f . bad1)
[BTW, "bad2" has the type of the Y combinator and indeed works as expected:
factorial :: (Int -> Int) -> Int -> Int
factorial _ 0 = 1
factorial r n = n * (r (n-1))
main :: IO ()
main = print $ map (bad2 factorial) [1..10]
... so one can get general recursion just by crafting such a strange data
type.]
--
Russell O'Connor <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''
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